An experimental study on the effect of joining interface condition on bearing response of single-lap, countersunk composite-aluminum bolted joints

An experimental study on the effect of joining interface condition (including shimming and interface gap) on bearing response of single-lap, countersunk composite-aluminum bolted joints are presented. The specimens consisted of a T700/3068 carbon/epoxy laminate with quasi-isotropic lay-up and an Aluminum alloy 7075T651 substrate. Bearing stress/bearing strain behavior were obtained according to ASTM standard. Both solid shim and liquid shim were considered and a comparison was made for them. 3D Digital Image Correlation was utilized to evaluate the effect of shimming on the surface strain distribution and out-of-plane deformation of the joints. One focus of the study was to investigate the effect of interface gap on the bearing performance of composite bolted joints. The interface gap was designed and characterized by variable parameters, i.e., thickness and span. It is found that compared to liquid-shim series, specimens with solid shim gain a little better bearing performance because of higher joint bending stiffness that benefits from the higher tensile modulus of solid peelable fiberglass shim. The presence of interface gap significantly weakens the bearing performance of single-lap, countersunk composite-aluminum joints by making the countersunk hole losing support from aluminum plate at the shear plane, and meanwhile intensifying the loading eccentricity of single-lap joints.